Final answer:
To calculate the mass percentage of an element, the mass of the element in the compound and the molar mass of the compound are required. The atomic number and the number of moles of the element are not necessary for this calculation.
Step-by-step explanation:
To calculate the mass percentage of an element in a given compound, it is necessary to have specific pieces of information. The mass of the element in the compound and the molar mass of the compound are two critical pieces. Knowing these allows one to use the formula:
% by mass = (mass of element in 1 mol × 100%) / molar mass of the compound
It is not necessary to know the number of moles of the element or the atomic number of the element for this particular calculation, although such information may be part of deriving the required mass. The percent composition of a compound is a reflection of the mass contributed by each constituent element relative to the total mass of the compound, and it remains constant as long as the compound is pure.